Thermal protection systems are employed in a wide variety of applications including, but not limited to, reusable launch vehicles and hypersonic vehicles. Such thermal protection systems are typically configured to provide a thermal shield for the vehicle substructure and the internal systems (e.g., fuel system, control system) and subsystems. For example, a thermal protection system (TPS) may be applied to a hypersonic vehicle to provide a thermal shield against extreme surface temperatures of the TPS. The extreme surface temperatures may result from aerodynamic heating due to hot convective flow passing over the exterior surfaces as the vehicle travels through the atmosphere at speeds of up to Mach 5 and beyond. The TPS must be capable of maintaining the vehicle substructure at temperatures below the temperature at which material properties of the substructure begin to degrade. In addition, the TPS must be capable of maintaining the internal systems at temperatures below their operational limits.
In addition to protecting the substructure and internal systems against excessive temperatures, a TPS must also accommodate movement of the TPS relative to the substructure. Such relative movement may result from mechanical loads that may be imposed on the vehicle and/or thermal stresses that may be imposed on the vehicle. Mechanical loading of the vehicle may occur due to aerodynamic forces acting on the vehicle during flight. Such aerodynamic forces may cause flexing of the vehicle and movement of the TPS relative to the substructure. Mechanical loading may also occur during launch of the vehicle or due to ground handling of the vehicle and which may also result in movement of the TPS relative to the substructure.
In addition to accommodating mechanical loads, the TPS must be capable of accommodating thermal stresses caused by differences in thermal growth of the TPS relative to the thermal growth of the underlying substructure. Such differential thermal growth may result from differences in the coefficient of thermal expansion (CTE) of the different TPS materials relative to one another and differences in the CTE of the TPS relative to the CTE of the substructure materials. Differential thermal growth may also occur as a result of differences in the temperature of the TPS and the relatively cooler temperature of the underlying substructure. Differential thermal growth may additionally occur as a result of dissimilar heating on different parts of the vehicle concurrently.
On a reusable launch vehicle such as the Space Shuttle, the TPS may comprise a plurality of individual tiles that may be attached to the substructure. One technique for accommodating relative movement of the substructure and TPS is to include gaps between the tiles. However, in order to restrict leakage of hot convective airflow into the gaps and to minimize aerodynamic pressure losses of the airflow passing over the gaps, it is necessary to minimize gap dimensions or fully seal the gaps. Minimizing gap dimensions between tiles may require forming the tiles with relatively tight dimensional tolerances. Unfortunately, for a relatively large quantity of tiles applied to a vehicle (e.g., tens of thousands on a single Space Shuttle), forming tiles with relatively small dimensional tolerances and sealing the tile gaps may significantly add to the cost and complexity of the TPS. Furthermore, in order to maintain aerodynamic smoothness of a vehicle with multiple-tile TPS, it is necessary to minimize surface steps at the gaps between the tiles to minimize the potential for localized heating and early transition from laminar to turbulent flow. The requirement to minimize surface steps further adds to the cost and complexity of the TPS.
A further drawback associated with individual tiles is that such tiles are typically adhesively bonded to the substructure. For a reusable launch vehicle, it is typically necessary to perform inspection, maintenance, and refurbishment operations on the vehicle between flights. In order to gain access to the substructure and the internal systems of the vehicle, it may be necessary to remove the tiles. Due to the adhesive bonding of the tiles to the substructure, removal of the tiles may be a time-consuming and labor intensive process and may result in damaging or destroying the removed tiles.
Furthermore, the installation of new tiles may require removal of the old adhesive and preparation of the surface of the substructure for new tiles. New tiles with appropriate geometry may then be fabricated, fit-checked to the vehicle, and adhesively bonded to the substructure. The process of bonding new tiles to the substructure may require the use of a vacuum bag to force the tiles against the substructure under pressure. The vacuum bag may be held under vacuum until the adhesive cures after which the vacuum bag may be removed. Seals may then be installed within the tile gaps followed by inspection of the replaced tiles and seals. As may be appreciated, the total process of removing and replacing tiles may be rather lengthy and may entail a significant amount of touch labor and vehicle down time.
As can be seen, there exists a need in the art for a TPS for a vehicle that may be easily removed from the vehicle to allow access to the substructure and internal systems in a manner that minimizes vehicle down time and avoids damage to the TPS. In addition, there exists a need in the art for a TPS that can accommodate movement of the TPS relative to the substructure due to mechanical loading of the vehicle and due to differential thermal growth of the TPS relative to the substructure.